This invention relates generally to power conversion method and apparatus in which a DC input voltage is converted to an AC output voltage of predetermined waveform shape and amplitude characteristics. More specifically, alternating polarity, modified square wave output voltages across a transformer's multi-tapped secondary winding are selectively switched to a common wired-OR junction where they combine to produce a regulated, source- and load-responsive, stepped approximation to a stored waveform representation. Series-connected, independently controlled switch devices forming switching circuits connected between each tap and the common junction are operable bidirectionally so that rectified surplus AC power concurrently recharges the DC source. A similar circuit is described that is operable as a charger only.
With high-frequency switching sine wave inverters, if the application requires typical operation into a fairly small load, but occasional operation into a very large load, one must buy a sine wave product rated for the large load. With square wave inverters, substantial surge capacity (e.g. three times the rated continuous power) is available, but there is insufficient peak voltage (contrast current) during such surges reliably to operate high inductance loads, e.g. motors. A square wave inverter typically generates little radio frequency interference (RFI) but has too high a total harmonic distortion (THD) (.gtoreq.50%) reliably to drive a typical AC load, which expects a `clean` sinusoidal input. A sine wave inverter has relatively low THD (.ltoreq.5%), but, because of the high switching frequency typically employed, generates an undesirably high amount of RFI.
It is a principal object of the present invention to provide power conversion apparatus that exhibits only moderate THD and that generates little RFI.
Another object of the invention is to provide inversion apparatus that produces an output AC voltage waveform that represents a stepped approximation to a sine waveform and the root-mean-squared (RMS) value of whose amplitude is substantially equal to the RMS value of the sine waveform that it approximates.
It is yet another object to provide such inversion apparatus with the ability to respond in real time to changing conditions of DC source or AC load.
Still another object is to use bidirectional switching devices alternately to impress a positive and negative DC voltage across the primary winding of a transformer and to use bidirectional switching devices connected in series with corresponding multiple taps of the secondary winding of the transformer, in order to provide not only inversion, but also concurrent charging of an DC element by an AC element.
Another important object is to provide charging apparatus that, like such inversion apparatus, includes a switched, multi-tapped transformer, as above.
Yet another object of the invention is to provide such apparatus in a form that is relatively easily and inexpensively manufactured.
The present invention solves the prior art problems by producing a high peak amplitude voltage, and delivers such a voltage when the load needs it the most, e.g. during start-up of a motor. It does so by providing an laternating polarity voltage to a center-tapped primary winding of a transformer, wherein the alternation is achieved by the programmable logic control of bidirectional switching devices connected between a negative battery terminal and either of the secondary winding's end taps. The transformer has a multi-tapped secondary winding, wherein the individual taps having predetermined turns ratios relative to the primary winding are selectively wired-OR'd by the logic-controlled, sequential operation of an array of switching circuits. The logic controller determines which of the switching circuits to operate at any given moment based upon which tap is currently connected, the calculated value representing the voltage across the secondary winding and the difference between the normalized, calculated voltage and a stored sine waveform representation. The result is a stepped approximation to a sine waveform that is highly responsive in real time to dynamically changing conditions of the DC source, e.g. battery sag, and the AC load, e.g. motor surge. In a modification to the invention, the apparatus operates as a battery charger only.
These and other objects and advantages of the invention will be more clearly understood from a consideration of the accompanying drawings and the following description of the preferred embodiment.